The high frequency of early deaths associated with breast cancer results from the recurrence of metastatic disease. The Breast Cancer Prognostic Study was organized to identify those characteristics of breast cancer patients and their tumors which are associated with early recurrence. Such markers would aid in targeting follow-up and therapeutic modalities to those at greatest risk; and systematic study of a large number of individual breast cancers and their hosts, with complete clinical follow-up, would provide an unprecedented view of the basic biology of breast cancer. Based on the active participation of the local clinical community, medical, surgical, and pathology associate groups, together with the Biological Resources Unit, are responsible for the recruitment of patients, clinical history, diagnostic evaluation, follow-up, recurrence data, and procurement of tumor, lymph node and blood specimens. A key element is the Pathology Panel which provides detailed pathological characterization of specimens. The clinical specimens are distributed to research laboratories where the materials are characterized via morphology, immunology, biochemistry, endocrinology, and cell biology. The assays are chosen on the basis of past and current data associating them with malignant potential, tumor burden or resistance to tumor development. As of Feb. 1, 1979, 461 patients had been entered into the Study, averaging 3-5 new entries per week.